Dr Murat Gunel, who specialises in surgical treatment of strokes, said a lack of adequate oxygen in Blaine's brain during the breath-holding challenge, especially after seven days under water, carried a risk of irreversible brain injury.

"David might never be the same again," he said.

"Even if he is fine immediately after, there is a risk for developing seizures in the long run.

"We all advised him against this stunt."

Blaine entered the "bubble" wearing trousers, rubber shoes and a special diving mask.

During the seven-day challenge the water temperature will be maintained at around 36C and he will be able to control his own core body temperature by wearing specially designed wetsuits.

He has also undergone intensive training for the challenge, shedding 50lbs (23kg) in body weight since January to improve the efficiency with which his body uses oxygen.